I can't really see what's wrong with the fish since I don't know which fin to look at xD And I can't find any black moor in the photos in the first place d: But I guess you mean the telescope eye there
Either way, if you say that it looks a bit moldy? I would say that it might be a fungal infection, possibly with some fin rot. So I would separate him and start treating with anti-fungal and -bacterial meds
You could try Melafix and Pimafix for now and see if it works. They are natural remedies so they're rather to help the fish recover by itself than to actually kill off any bacteria. You could try these for a few days, and if they don't work, try buying something stronger, still for bacterial/fungal disease.
If you want, you could add some salt along with the Melafix and Pimafix
Salt works pretty well against fungus too, and it's like a tonic for the fish